The need for a sharpening device designed for the sharpening of a carpenter's pencil has long been well known, as sharpening has generally required the use of a knife to accommodate the odd shape of the pencil. The difficulty of sharpening this style of pencil with such a sharpening device without leaving wood on the broad sides of the rectangular pencil lead has long presented a major obstacle. For example, in U.S. Pat. No. 5,077,903 a sharpener is presented with dual rotary cutter assemblies arranged to cut two curves in the point. In U.S. Pat. Nos. 4,759,129 and 4,918,816 four cutters and associated gearing are similarly used to achieve the desired sharpening effect on a carpenter's pencil. In U.S. Pat. No. 4,081,010 a cutting blade is driven around the pencil in a complicated “eccentric” motion by means of cams to achieve the required pencil point shaping. All of these references, however, require complex configurations with many parts such as blades, gears and motors.
One solution is presented by U.S. Pat. No. 6,092,293 issued to Donaldson. Donaldson discloses the use of a curved or bent blade which enables the blade to more closely form an ideal contour of the carpenter's pencil as it is rotated. The curved or bent blade removes enough wood to expose a good writing pencil tip but not too much wood resulting in a weak tip that could break with little pressure.
The sharpener from the '293 patent uses a snap-fit, rotatable centering collar that is attached to a housing that also has a concave-cone surface that holds the blade. The centering collar has a hole that is sized to match octagonal edges of the non-circular pencil, completely fixing the pencil within the centering collar so that the centering collar rotates with the pencil. The centering collar maintains the pencil at a centered position for sharpening. Otherwise a misalignment of pencil to blade and cone surface can cause lead breaks and jamming.
Another similar solution that copies the '293 design is presented by U.S. Pat. No. 6,571,480 issued to Qui that presents a more complicated, rotatable centering collar that is also rotatably attached to a blade housing. The collar is locked in a chamber of the sharpener by a separate removable plastic clip that is prone to breaking when removing the clip to clean the sharpener.
However, because a moving part, the rotatable centering collar, is required, the sharpener will not work if something (e.g. dust or debris) jams the collar so that it cannot rotate. In addition, because the centering collar is a separate part that requires it own molding/manufacturing process and then an assembly process for attachment to the main housing, it adds relatively significant production cost. Thus, another more economical alternative solution may be desired.
Another problem with carpenter's pencil sharpeners is that while the sharpener cuts well when the grain of the wood on the pencil is parallel to the blade, when the blade is cutting the pencil crossgrain, it takes more force to rotate the pencil on the blade, and the straight edge of the blade often binds, causing choppiness (jumping of the pencil on the blade) leading to frequent breaks and jams. Thus, a carpenter's pencil sharpener that cuts well, no matter the direction of the wood grain on the carpenter's pencil, is desired.